1. Field of the Invention
The present invention relates to an acoustooptic device, and particularly to an acoustooptic device which is preferably used as a light deflecting device for a laser printer or other suitable device.
2. Description of the Related Art
FIG. 7 is a perspective view showing a conventional acoustooptic device, and FIG. 8 is a diagram showing a main part of the acoustooptic device of FIG. 7. The acoustooptic device deflects light beams by using an interaction between light and ultrasonic waves such as surface acoustic waves. The acoustooptic device 1 includes a LiNbO.sub.3 layer 2 of Y-cut, and an optical waveguide layer 3 which is formed of a thin film of Nb.sub.2 O.sub.5 is located on the LiNbO.sub.3 layer 2. An input grating 4 is located on the principal plane at a light input side of the optical waveguide layer 3 so as to be substantially perpendicular to a light incident direction. An output grating 5 is located on the principal plane at a light output side of the optical waveguide layer 3 so as to be substantially parallel to the input grating 4. The input grating 4 and the output grating 5 collect spatial light beams into the optical waveguide and combine the spatial light beams. The input grating 4 and the output grating 5 are formed as plural grooves which are parallel to one another, or formed of plural rod-shaped electrodes which are parallel to one another.
Further, an interdigital electrode 6 is located on the principal plane of the optical waveguide layer 3 so that a Rayleigh wave, which is a kind of surface acoustic wave, is excited at the intermediate portion between the input grating 4 and the output grating 5. The interdigital electrode 6 is formed of a pair of comb-shaped electrodes 6a and 6b which are mutually inserted into each other (or interdigitated) as shown in FIG. 8. One comb-shaped electrode 6a is grounded while the other comb-shaped electrode 6b is connected to an oscillator for applying a frequency.
In the acoustooptic device 1, a Rayleigh wave is excited so as to have a frequency corresponding to a frequency applied by the interdigital electrode 6. One light beam is incident from a light source 7 into the optical waveguide layer 3 of the acoustooptic device 1. The light beam which is incident from the light source 7 into the optical waveguide layer 3 is diffracted by the Rayleigh wave which is excited by the applied frequency, so that a different light diffraction (deflection) angle can be obtained by changing the frequency applied to the interdigital electrode 6. The acoustooptic device 1 may be used as a light deflection device for a laser printer or the like.
The variation (variable width) .DELTA..theta. of a light deflection angle at which the light beam is diffracted in accordance with the variation .DELTA.f of the frequency is represented by the following equation: EQU .DELTA..theta.=(.lambda..sub.0 .about..DELTA.f)/(2v.multidot.cos .theta..sub.B) (1)
Accordingly, the variation .DELTA..theta. of the light deflection angle is dependent on the frequency variation .DELTA.f of the surface acoustic wave. In order to set .DELTA.f to a large value, it is better to broaden the frequency band of the surface acoustic wave. In order to efficiently excite the surface acoustic wave and thus broaden the frequency band, an electromechanical coupling factor K must be set to a large value.
However, the conventional acoustooptic device as described above has a small electromechanical coupling factor K. For example, in the case of a Rayleigh waveform of a LiNbO.sub.3 substrate of Y-cut and Z-propagation, K is theoretically equal to 0.22, and in the case of Rayleigh waveform of a 128.degree.-rotating Y-plane LiNbO.sub.3 substrate, K is theoretically equal to 0.23. Therefore, in the conventional acoustooptic device, it is difficult to broaden the frequency band .DELTA.f of the frequency at which the Rayleigh wave is excited, and it has been impossible to increase the variation (variable width) of the light deflection angle.